The invention relates to a semiconductor device comprising a wafer-shaped semiconductor body having a surface which is at least partly covered with an electrically insulating layer and comprising at least a transistor having at least a base zone of a first conductivity type which adjoins the said surface, a collector zone of the second conductivity type which adjoins the base zone and constitutes a collector-base junction with the base zone, and likewise surface-adjoining emitter zone of the second conductivity type. The emitter zone and base zone form an emitter-base junction terminating at the surface, the emitter zone and the base zone each having an electrode layer which adjoins the relevant zone within a contact surface. The device further includes a first electrode layer and the associated first contact surface surrounding the other, second electrode layer substantially entirely, with, at most, electrode layers which are connected to the semiconductor surface being present above the part of the semiconductor surface between the inner edge of the first contact surface and the oppositely located edge of the second contact surface associated with to the second electrode layer.
The invention also relates to a very advantageous method of manufacturing a device according to the invention and to a circuit arrangement which comprises such a device.
It is known that in bipolar transitors (and this is of course of particular importance for power transistors which are operated at high currents and voltages) a phenomenon may occur which is known as second breakdown. In certain circumstances, current concentration in certain regions of the emitter and/or the base zone occurs which may result in local overheating and irreparable damage.
Several measures have already been proposed to check the phenomenon of second breaKdown and, if possible, to avoid it entirely. The object of all these measures is that the current which passes through the emitter-base junction is not restricted to only a small part of the total emitter region or the total base region, but will distribute uniformly over the whole emitter-base junction.
The best known of said measures consists in providing a distributed series resistor between the emitter connection and the emitter-base junction, in which a part of the emitter zone which locally draws too much current comes to a lower emitter-base voltage as a result of the distributed series resistor and will hence take part in the injection to a smaller extent. In many power transistors the emitter is subdivided into partial zones in which case each of said partial zones must be provided with a series resistor so as to obtain the said effect of a distributed emitter-series resistor.
Instead of, or besides, using a distributed emitter-series resistor, second breakdown may also be prevented, or at least the danger thereof can be reduced, by using a distributed base-series resistor between the base electrode layer and the emitter-base junction. Generally such a base-series resistor, however, will be less effective, that is to say, it will have to have a higher value than a corresponding emitter-series resistor, since the base current is only a fraction of the total emitter current and, in order to obtain an effective effect across the base resistor, it will be necessary to obtain a voltage drop of the same order of magnitude as across a comparable emitter-series resistor. The manufacture of an effective divided base resistor of comparatively high resistance meets with several technological and geometrical difficulties.
On the other hand, it will, in certain circumstances, nevertheless be preferred to use a distributed base-series resistor instead of a distributed emitter-series resistor. This is the case, for example, when the transistor is incorporated in a so-called common emitter circuit in which the emitter zone is set up at a fixed reference potential, for example earth.
One of the objects of the present invention is to provide a construction for a semiconductor device having a transistor with incporated divided base resistor which can be manufactured in a very simple manner and moreover shows electrically important advantages.